Sorry to hear that you got hijacked. The first thing you will have to do is change your Steam account password as well as the email that you use for Steam Guard. You'll them need to scan your computer for any malware and forwarding email addresses that may have been inserted without your knowledge. Please check your Steam Guard Email to see if your account was accessed from another location. Try and remember if you clicked on suspicious links, or somehow got phished.
A Steam support ticket should be immediately sent to Valve describing what happened and including all screenshots and information you think will help. Steam Rep cannot restore your items, as SR is not associated with Valve, but Valve can if your account was indeed hijacked.
-link to starting a steam support ticket- https://support.steampowered.com/newticket.php
Create a Steam Support account and send a ticket. I would recommend setting the product as *STEAM*, the category as "Account Questions" and the sub-category as "Hijacked or stolen account". Fill out the support ticket with as much information as possible.
To find the account that your items were traded to, check your inventory history. A screenshot of your trade history (full page) will be required for your report.
-link to - How to view your inventory history - http://pastebin.com/2SRRPey7
A screenshot of your support ticket (with the appropriate information edited out) will be needed to confirm that you have regained control of your account and that Valve has looked into the matter. Valve will reset your account password and send you an email regarding your account.
Please read the section on Account Hijackings here http://forums.steamrep.com/threads/howto-provide-evidence-with-screenshots.3488/.
A guide on how to prevent hijackings adn scan your computer can be found here - http://forums.steamrep.com/threads/guide-to-prevent-hijacking.5437/